Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as it is being manufactured. Generally, on-line measurements of sheet properties, such as thickness, gloss or smoothness are made by scanning sensors that travels back and forth across the width of the sheet of paper in the cross-machine direction (CD). The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices.
In conventional papermaking machinery, the sheet of material being manufactured tends to shrink in the cross-machine direction as it travels through the papermaking machinery. This is particularly true at the stage where the sheet passes through drying equipment. This shrinkage is not uniform across the sheet and, therefore, it is important to be able to establish a shrinkage profile across the sheet. Due to non-uniform shrinkage of the sheet, a downstream sheet slice that is measured a distance in from the edge of the sheet may be adjusted by activating an upstream actuator that is a significantly different distance in from the edge of the sheet. It is important to be able to establish the relationship between each downstream slice where scanning measurements occur and the corresponding upstream actuator that must be adjusted to control the particular downstream slice.
Identifying the shrinkage profile across a sheet as it passes from the formation process through dryers is a requirement for precise control of the sheet properties across the sheet. Traditional techniques for establishing a shrinkage profile have relied on manual tests where the sheet is marked with dye at an upstream location and the location of the dye is then manually measured at a downstream location after the drying process. The test is repeated at various locations across the width of the sheet. Based on the spacing of the measured downstream dye marks when compared with the known upstream dye positions, a shrinkage profile can be determined. This test requires considerable manual effort to visually identify the centre of each dye mark and relate it to a physical location on the sheet which is travelling by the observer at great speed. The accuracy of the shrinkage profile is compromised by manual or calculated measurement precision. Dye must be applied for a sufficient period to allow the observer to locate and measure all mark centres which often means making dyed sheet paper for several minutes. The dyed sheet paper produced is not saleable.
Alternatively, bump tests can be used to determine the shrinkage profile which involves adjusting specific upstream actuators across the sheet and measuring the location in the downstream sheet after the drying process where a response is detected. The responses must be identified from within inherent process variability which is often quite significant near the sheet edges (due to wave or localized edge process variation) where much of the shrinkage occurs. The bump response centres are often difficult or impossible to identify close to the sheet edge particularly on heavyweight profiles due to localized variability. Furthermore, bump tests often take 30 minutes to an hour to complete due to averaging necessary to determine the true response. During this period the sheet material produced is effectively wasted since it is not saleable.